This invention relates to control cables and more particularly to an improved straned conduit for a control cable.
Control cables are used for a wide variety of applications. A typical cable assembly comprises a reinforced tubular conduit and an interior core wire that slides longitudinally inside the conduit. The core wire may be a multi-stranded cable, which is primarily used for tension applications, or it can be a solid wire, which is used for push-pull applications.
A conventional conduit for a cable assembly comprises a plastic inner liner designed to provide good slip characteristics with respect to the core wire; a wire reinforcement layer; and an abrasion resistant jacket extruded or otherwise formed over the reinforcement layer.
The reinforcement layer can be formed in several ways. The reinforcement layer can be an open weave braided cable; it can be a closed helix formed of a single strand of flat or round wire; or it can be a so-called "stranded" conduit, wherein a plurality of parallel, wire strands are wrapped in an open or closed helical fashion around the liner at a predetermined "pitch". The term "pitch" refers to the slope of the helix with respect to a line perpendicular to the axis of the helix and tangent to the exterior surface of the helix. The value of the pitch can be expressed as a slope angle or it can be defined (more commonly) as the axial length of one complete loop of the helix of one wire strand. Both types of measurements are used herein.
Stranded conduit is widely used in a number of applications, including automotive control cables. In one common construction of such a cable, the wire strands or lay wires in the reinforcement layer are formed of a hardened steel known as oil tempered wire or low grade music wire having a carbon content of about SAE 1065. The construction typically employs twenty (20) strands or less, with each strand being at least 0.016 inches in diameter. The strands can be wound in an open or closed helical fashion at a wide range of pitches. The high carbon steel reinforcement layer in such conduit makes such cables resistant to crimping or kinking upon bending and resists denting when the cable is subjected to a crushing force. The reinforcement layer thus makes it possible for the core wire to continue to slide freely in the conduit, even when the conduit is hit or bent sharply.
Heretofore, it has been believed that high carbon steel is essential in order to provide sufficient protection for the core wire. However, the high carbon steel has several drawbacks, in particular hardened steel conduit lacks a high degree of flexibility and is expensive.
The specifications for certain cable assemblies used in automotive applications require such a high degree of flexibility, combined with performance, deformation, and cost criteria, that conventional high carbon steel stranded conduit cannot meet all criteria of these specifications.
It is an object of the present invention to provide an improved stranded conduit that has adequate strength and improved flexibility and yet is less expensive than conduit employing hardened steel wire.